Transverse utrasound peening of blades on a rotor

ABSTRACT

The invention concerns a method for transverse ultrasound peening of blades ( 2 ) on a rotor which consists in driving in rotation the wheel ( 3 ) bearing the blades ( 2 ) about its geometrical axis ( 6 ) arranged substantially vertically and in causing the blades ( 2 ) to pass through a mist of microbeads produced by a vibrating surface ( 20 ) in an active chamber ( 12 ) arranged laterally relative to the wheel. The active surface ( 20 ) is located beneath the path of the blades ( 2 ). Preferably, the active chamber ( 12 ) comprises a second vibrating surface above the path of the blades ( 2 ). The invention also concerns a machine for implementing said method.

[0001] The invention relates to a method for the ultrasonic peening ofparts which lie radially at the periphery of a wheel, such as theaerofoil sections of turbomachine blades on a rotor. The invention alsorelates to a peening machine for implementing the method.

[0002] The term “wheel” is to be understood as meaning an object theoverall shape of which exhibits symmetry of revolution about a geometricaxis and which can be rotated about its axis.

[0003] In order to improve the fatigue strength of mechanical parts, itis known practice for their surface to be peened using microbeads. Thistechnique is very widely used in the aeronautical industry to place thesurface of parts under permanent compression to a shallow depth. Thisintroduction of compression opposes the onset or growth of cracks at thesurface of the part, and this makes it possible to improve the fatiguestrength. The technique consists in propelling microbeads against thesurface of the part at an angle of incidence which is small with respectto the normal to this surface and with sufficient kinetic energy.

[0004] As a preference, the angle of incidence is below 45° with respectto the normal to the surface so that the impact can transmit sufficientenergy from the bead to the impacted surface. Exposure of the part topeening passes through an optimum. Insufficient peening does not yieldthe anticipated strength but additional peening can still be performed.On the other hand, excessive peening causes irreversible damage to thepart.

[0005] The shot peening technique is applied in particular forcompressing the surfaces of the aerofoil sections of the blades of aturbomachine rotor. In the case of fine-walled blades, it is necessaryto peen both sides of the aerofoil sections at the same time, so as toavoid deformation through modification of the curvatures in the thinregions.

[0006] Traditionally, thick-walled surfaces are peened by propelling themicrobeads using a nozzle fed simultaneously with compressed gas andwith microbeads. The aerofoil sections of turbomachine blades are peenedby means of two nozzles each peening one side of the aerofoil section.This method of peening in itself has two drawbacks:

[0007] the peening parameters are not stable, and the peening machinehas frequently to be checked and adjusted when seeking close to optimumpeening,

[0008] the surface finish is damaged, which detracts from the life ofthe parts,

[0009] the method has to be carried out in a cabin which is large enoughto allow the parts and the peening nozzles to be manipulated.

[0010] When the surfaces for peening are the aerofoil sections ofone-piece bladed wheels, separated by relatively small distances, thepeening method using nozzles is even more tricky to implement.

[0011] In the French patent application filed on Nov. 18, 1999 andrecorded under the number FR 99 14 482, the applicant company proposed amethod of ultrasonic peening using a mist of microbeads sustained in anactive chamber by a vibrating surface. According to the method describedin that application, the wheel is rotated about its axis which isarranged horizontally. The blades situated in the lower part of thewheel pass through the active chamber at low speed and are impacted bythe microbeads of the mist sustained by the vibrating surface arrangedunder the ends of the lower blades.

[0012] The microbeads activated by the vibrating surface strike thesurfaces of the blades which are situated in the active chamber, offwhich they rebound, and the peripheral walls of the wheel which liebetween the blades. The microbeads which have lost their kinetic energydrop down onto the vibrating surface which propels them back into theactive chamber. Some microbeads leave the active chamber and arecollected in adjacent inactive chambers from where they return to thebottom of the active chamber under gravity.

[0013] The thin ends of the aerofoil sections are subjected to veryviolent impacts and have to be trimmed at the end of the peeningoperation.

[0014] During the peening operation, the wheel rotates through severalrevolutions. It is thus easier to reach the optimum and avoidasymmetries in peening, which asymmetries give rise to deformation whenthe parts are thin.

[0015] The method described in FR 99 14 482 is particularly suited tothe aerofoil sections of blades of relatively short length.

[0016] However, when the aerofoil sections are long by comparison withthe distance between two consecutive aerofoil sections, particularly ifthe ratio between the length and the interblade distance is greater thanthree, or alternatively when the height of the aerofoil section isgreater than 100 mm and the aerofoil section has a very curved shape,the flanks of the aerofoil sections situated toward the bottom of theinterblade space are not peened as much because the microbeads havealready rebounded several times in order to reach them and have lostsome of their kinetic energy. Thus, peening is not homogeneous and theduration of the peening has to be increased in order to make sure thatall points undergo a minimum amount of peening.

[0017] The object of the invention is to propose a method for theultrasonic peening of parts which lie radially at the periphery of awheel and which allows the surfaces of these parts to be peenedeffectively irrespective of their length.

[0018] The invention therefore relates to a method for the ultrasonicpeening of parts lying radially at the periphery of a wheel, accordingto which method the wheel is set in rotation about its geometric axisand a mist of microbeads is created in a fixed active chamber arrangedto the side of said wheel, by means of a first vibrating surfacearranged in the lower part of said active chamber comprising openingsshaped to allow the parts to be put in and removed as the wheel rotatesand being sized to accommodate at least three adjacent parts.

[0019] The method according to the invention is characterized in thatthe wheel is rotated about its axis, which is arranged roughlyvertically, and in that the first vibrating surface is arranged underthe path of the parts in the active chamber.

[0020] This arrangement allows all the surface regions of the partspassing through the active chamber to be impacted irrespective of theirdistance from the axis of rotation of the wheel.

[0021] According to an advantageous feature of the method according tothe invention, the chamber comprises a second vibrating surface abovethe path of the parts in the active chamber.

[0022] By virtue of this feature, the microbeads which reach the upperpart of the chamber with low kinetic energy and are ready to drop downunder gravity, are reactivated by this second vibrating surface, andonce again participate in the actual peening operation by rebounding offthe surfaces of the parts and the walls of the active chamber.

[0023] When the method according to the invention is applied to partshaving thin edges facing a vibrating surface, such as the leading edgesand trailing edges of the aerofoil sections of turbomachine blades, andaccording to another advantageous feature of the invention, said thinedges are protected during peening.

[0024] This protection may preferably be afforded by rods which rotateas one with the wheel and each conceal a thin edge. These rods arearranged between the thin edges and the sonotrodes. They have the effectof reducing the energy of the beads likely to impact the thin edges.They may be in contact with the thin edges or may be a small distanceaway therefrom.

[0025] It may also be afforded by fixed rods secured to the chamber. Inthis case, the wheel is rotated step by step during peening so that theedges of the parts lying in the active chamber face the fixed rods.Peening may be halted while the wheel is being pivoted by one step.

[0026] Thus, during peening, the rods lie between the thin edges of theblades and the sonotrodes so as to protect the thin edges fromhigh-energy impacts from balls coming directly from a sonotrode.

[0027] The invention also relates to a peening machine for implementingthe abovementioned method.

[0028] This machine is characterized in that it comprises:

[0029] a turntable of roughly vertical axis equipped with means forholding a wheel radially comprising parts for peening, coaxially withrespect to said turntable,

[0030] means for rotating the turntable about its axis, and

[0031] at least one device for peening said parts, said peening devicecomprising:

[0032] an active chamber arranged to the side of said wheel and sized tohouse at least three adjacent parts and having an opening shaped toallow the parts to be put in and removed as the region rotates,

[0033] a first vibrating surface arranged in the bottom of the activechamber under the path of the parts in said active chamber and able tosustain a mist of microbeads in said active chamber, and

[0034] means for collecting the microbeads which escape from the activechamber and returning them to said chamber.

[0035] Advantageously, the peening device further comprises a secondvibrating surface arranged in the active chamber above the path of theparts.

[0036] The machine may also comprise means for protecting the edges ofthe parts situated facing a vibrating surface.

[0037] Other advantages and features of the invention will becomeapparent from reading the following description given by way of exampleand with reference to the appended drawings in which:

[0038]FIG. 1 is a schematic view from above of a peening machineaccording to the invention on which is mounted a bladed turbomachinewheel the aerofoil sections of the blades of which need to be peened,

[0039]FIG. 2 is a vertical section on FIGS. II-II of FIG. 1;

[0040]FIG. 3 shows the fixing of the bladed wheel to the turntable ofthe machine and the arrangement of the arrays of gratings for protectingthe leading edges and trailing edges of the aerofoil sections;

[0041]FIG. 4 is a section of the peening machine on a vertical planeintersecting the plane of FIG. 1, on the line IV-IV;

[0042]FIG. 5 is similar to FIG. 4 and shows, on a larger scale, theactive chamber and the chambers for collecting the microbeads that leavethe active chamber;

[0043]FIG. 6 is a section on the line VI-VI of FIG. 4, in a horizontalplane passing through the chambers and situated under the path of theblades in the peening device; and

[0044]FIG. 7 is similar to FIG. 2 and shows, on a larger scale, thepeening device and the rods protecting the leading edges and trailingedges of the aerofoil sections, these rods being fixably mounted on thechambers.

[0045] In the drawings, the reference 1 denotes a machine for peeningthe aerofoil sections 2 which lie radially at the periphery of a wheel 3of axis x of a turbomachine. The wheel 3 may, for example, be aone-piece bladed disk (blisk) or a turbomachine wheel equipped withmoving blades. The aerofoil sections 2 may also be parts the surfaces ofwhich need to be peened and which comprise means for holding themradially and uniformly angularly spaced at the periphery of a wheel 3which then acts as a support for the parts that are to be peened.

[0046] The peening machine 1 essentially comprises a turntable 4 carriedby a shaft 5 of roughly vertical axis 6. The shaft 5 can be rotatedabout its axis 6 by rotational drive means, for example an electricmotor, not shown in the drawings. The wheel 3 is fixed to the turntable4 by means of a clamping piece 7 collaborating with a tapped bore 7 a ofaxis 6 formed in the turntable 4 so that its axis x coincides with theaxis 6 of the turntable 4.

[0047] As a preference, as can be seen in FIGS. 2 and 3, a first annularflange 8 is inserted between the turntable 4 and the wheel 3, and asecond annular flange 9 is inserted between the wheel and the clampingpiece 7.

[0048] These annular flanges 8 and 9 comprise, at their periphery,radial rods 8 a and 9 a respectively, equal in number to the number ofaerofoil sections 2 on the wheel 3, uniformly spaced about the axis x.Each rod 8 a and 9 a adopts the shape of the trailing edges and leadingedges of the aerofoil sections 2. The low annular flange 8 is positionedunder the wheel 3 in such a way that the array of radial rods 8 a coversthe lower edges of the aerofoil sections 2. The upper annular flange 9is also positioned angularly with respect to the wheel 3 in such a waythat the array of rods 9 a covers the upper edges of the aerofoilsections 2. As the turntable 4 rotates about the axis 6, the wheel 3 andthe annular flanges 8 and 9 rotate about the axis 6.

[0049] The diameter of the turntable 4 is chosen to suit the wheel 3 andsuch that the aerofoil sections 2 project radially from the periphery ofsaid turntable.

[0050] In FIGS. 1 to 3 it can be seen that the machine 1 also comprisesa fixed, roughly horizontal slideway 10, secured to the structuresupporting the shaft 5, and the axis of which is perpendicular to theaxis 6 of the shaft 5.

[0051] Mounted to slide on the slideway 10 is the actual peening device11 proper. When the wheel 3 is mounted on the turntable 4 or removedtherefrom, the peening device 11 is moved away from the turntable 4.

[0052] This peening device 11 essentially comprises a central chamber 12known as an active chamber arranged between two side chambers 13 and 14known as inactive chambers and intended to collect microbeads 15 whichmight escape from the central chamber and to return them to the centralchamber 12 as explained later on in this text.

[0053] The chambers 12 and 13 and 14 are delimited together by a rigidexternal peripheral wall 16 in the form of a circular sector and theinside diameter of which is roughly equal to or slightly greater thanthe diameter of the path followed by the tips of the aerofoil sections 2as the wheel 3 rotates about the axis 6, a dished lower wall 17 whichruns between the peripheral wall 16 and the periphery of the turntable 4and an upper wall 18 in the shape of an inverted dish or of a dome whichruns between the peripheral wall 16 and the periphery of the upperflange 9.

[0054] The lower wall 17 is arranged under the path followed by theaerofoil sections 2 as the wheel 3 rotates and the upper wall 18 issituated above this path. A lower vibrating surface 20 is arranged inthe bottom of the dish formed by the lower wall 17 and a secondvibrating surface 21 is arranged in the upper part of the dome formed bythe upper wall 18.

[0055] Vertical and radial partitions with openings the outline of whichis shaped according to the annular surfaces generated by the rods 8 aand 9 a as the wheel 3 rotates, connect the walls 17 and 18 to theperipheral wall 16. These partitions, of which there are four above andbelow the path of the aerofoil sections 2 comprise, in particular,lateral end partitions 21 a, 21 b which circumferentially delimit theinactive chambers 13 and 14, and intermediate partitions 22 a, 22 bwhich separate the active chamber 12 from the inactive side chambers 13and 14. The lower intermediate partitions 22 a, 22 b have, near thelower wall 17, openings or slots 23 which allow the microbeads 15 whichenter the inactive side chambers 13 and 14 to return to the lowervibrating surface 20 under gravity.

[0056] The active chamber 12 is thus circumferentially delimited by thepartitions 22 a and 22 b and is arranged between the vibrating surfaces20 and 21 as visible in FIG. 5.

[0057] The circumferential size of this active chamber 12 is such thatat least three aerofoil sections 2 can be housed in this active chamber12.

[0058] A certain amount of microbeads 15 is placed in the active chamber12. When the vibrating surfaces 20 and 21 of the sonotrodes areactivated, the microbeads 15 placed above the lower vibrating surface 20are propelled upward, strike the surfaces of the aerofoil sections 2,rebound off these surfaces and continue on their way randomly. Some ofthese microbeads 15 reach the upper vibrating surface 21 which givesthem further kinetic energy. These beads 15 once again strike the wallsof the blades 2 as they descend. It goes without saying that somemicrobeads 15 strike the intermediate partitions 22 a and 22 b off whichthey rebound. These microbeads 15 remain in the active chamber 12 anddrop back onto the vibrating surface 20 when they have lost theirkinetic energy.

[0059] Because of the movement of the aerofoil sections 2 through theopenings formed between the upper and lower intermediate partitions 22 aand 22 b, some microbeads 15 enter the side chambers 13 and 14 via thespace separating the contours of the partitions 22 a and 22 b from theclosest rods 8 a and 9 b. These microbeads 15 quickly lose their kineticenergy in the side chambers 13 and 14, drop onto the bottom wall 17which is inclined, and return to the lower vibrating surface 20 via theslots 23 formed at the foot of the lower intermediate partitions 22 aand 22 b.

[0060] As the wheel 3 rotates through one revolution, the aerofoilsections 2 are impacted by the microbeads 15 for the time that they areresident in the active chamber 12.

[0061] Advantageously, this residence time is markedly shorter than thetotal peening time needed to obtain the optimum result, and the numberof revolutions to be performed in order to obtain the optimum result iscalculated accordingly. This number of revolutions is at least equal to3. This makes it possible to reduce the deformation of the aerofoilsections as a result of the temporary differences in peening between thetwo faces of the aerofoil sections during treatment. What happens isthat when an aerofoil section enters the chamber, its face facing in thedirection of rotation experiences more intense peening than its oppositeface, because it is more exposed to the high-energy impacts of the beadscoming directly from the sonotrode. The compressive preloading of theforward-facing face is therefore greater than that of the opposite face,which causes partially plastic deformation toward the rear of theaerofoil section. When the aerofoil section is leaving the peeningchamber, it is the opposite phenomenon which occurs, but residualaerofoil-section deformation nonetheless remains.

[0062] By carrying out the peening over N revolutions instead of justone, the temporary difference in peening between the two faces of theaerofoil sections is divided by N, which divides the resultantdeformation of the aerofoil sections more or less by N. The number N ofrevolutions is not critical. Three to five revolutions is considered bythe applicant as being acceptable for obtaining a significant result.

[0063] It should be noted that in order to reduce the total peening timeit is possible to equip the machine 1 with several peening devices 11identical to the one described hereinabove and which are distributedangularly about the axis 6.

[0064]FIG. 7 shows an alternative form of embodiment of the system forprotecting the leading edges and trailing edges of the aerofoil sections2. In this alternative form, the annular flanges 8 and 9 do not compriseany arrays of radial rods 8 a, 9 a. The protective rods 30 and 31, whichare fixed with respect to the peening device 11, are mounted in theactive chamber 12. The number of rods 30 and 31 is equal to the numberof aerofoil sections 2 that can be housed in the active chamber 12.

[0065] During the peening operation, the aerofoil sections 2 areimmobilized for a certain length of time in a position such that theirleading edges and their trailing edges are protected by the rods 30 and31. They are then moved through a step equal to the angular spacingbetween two consecutive aerofoil sections 2.

[0066] In a preferred embodiment of the invention, the rods 30, 31 arefixed, at one end 32, 33, to the outer wall 16 and, at the other end, toa common support 34, 35 which acts as a seal between the rotor 3 and,respectively, the interior walls 17, 18, this seal being afforded whenthe clearances left are smaller than the diameter of the beads.

[0067] To make it easier to get the rotor 3 into the peening chambers12, 13 and 14, it may be advantageous for the outer wall 16 to be splitinto two parts 16 a and 16 b separated by a parting line 36 more or lessin the plane of the rotor 3. The rotor is then introduced using thefollowing procedure:

[0068] moving apart, along the path 37, the upper constituents of thechambers, namely the upper part 16 a of the outer wall 16, the sonotrode21 and the internal wall 18,

[0069] introducing the rotor 3 along the path 38,

[0070] bringing back together these same upper constituents of thechambers along a path 39 that is the opposite of the path 37, so as toclose the chambers again around the rotor and allow peening to takeplace.

[0071] This step by step movement is performed at high speed if peeningcontinues during this movement, so that the leading edges and thetrailing edges are impacted infrequently during the movement. It is alsopossible to shut down the sonotrodes for the time that the aerofoilsections 2 are being moved stepwise.

1. A method for the ultrasonic peening of parts (2) lying radially atthe periphery of a wheel (3), according to which method the wheel (3) isset in rotation about its geometric axis (6) and a mist of microbeads(15) is created in a fixed active chamber (12) arranged to the side ofsaid wheel (3), by means of a first vibrating surface (20) arranged inthe lower part of said active chamber (12), said active chamber (12)comprising an opening shaped to allow the parts (2) to be put in andremoved as the wheel (3) rotates and being sized to accommodate at leastthree adjacent parts, characterized in that the wheel (3) is rotatedabout a roughly vertical axis (6) and in that the first vibratingsurface (20) is arranged under the path of the parts in the activechamber (12).
 2. The method as claimed in claim 1, characterized in thatthe active chamber (12) comprises a second vibrating surface (21) abovethe path of the parts (2) in the active chamber (12).
 3. The method asclaimed in one of claims 1 or 2 applied to parts (2) having thin edgesfacing a vibrating surface (20, 21), characterized in that said thinedges are protected during peening.
 4. The method as claimed in claim 3,characterized in that the thin edges of the parts (2) are protectedusing rods (8 a, 9 a) which rotate as one with the wheel (3).
 5. Themethod as claimed in claim 3, characterized in that the thin edges ofthe parts (2) situated in the active chamber (12) are protected usingrods (30, 31) secured to the chamber (12) and the wheel (3) is rotatedstep by step during peening.
 6. The method as claimed in any one ofclaims 1 to 5, characterized in that the wheel performs at least N=3revolutions during peening.
 7. A peening machine for implementing themethod as claimed in claim 1, characterized in that it comprises: aturntable (4) of roughly vertical axis (6) equipped with means forholding a wheel (3) radially comprising parts (2) for peening, coaxiallywith respect to said-turntable (4), means for rotating the turntable (4)about its axis (6), and at least one device for peening said parts (2),said peening device comprising: an active chamber (12) arranged to theside of said wheel (3) and sized to house at least three adjacent parts(12) and having an opening shaped to allow the parts (12) to be put inand removed as-the wheel (3) rotates, a first vibrating surface (20)arranged in the bottom of the active chamber (12) under the path of theparts (2) in said active chamber and able to sustain a mist ofmicrobeads (15) in said active chamber (12), and means for collectingthe microbeads (15) which escape from the active chamber (12) andreturning them to said chamber (12).
 8. The machine as claimed in claim7, characterized in that the peening device further comprises a secondvibrating surface (21) arranged in the active chamber above the path ofthe parts (2).
 9. The machine as claimed in one of claims 7 or 8,characterized in that it further comprises means for protecting theedges of the parts (2) situated facing a vibrating surface (20, 21). 10.The machine as claimed in claim 9, characterized in that the protectingmeans comprise an array of radial rods (8 a, 8 d) secured to the wheel(3).
 11. The machine as claimed in claim 9, characterized in that theprotective means comprise rods (30, 31) secured to the active chamber(12).
 12. The machine as claimed in any one of claims 7 to 11,characterized in that the peening means can move in a direction roughlyperpendicular to the axis (6) of the turntable (4).